The non-plant biofilter should contain materials that will not readily decompose, will not get compacted and yet allow lots of surface area (small interstitial openings) and air spaces for colonization of denitrifying bacteria. Light weight material would be helpful but not necessary.
Simplest material would be stones the size of golf balls or large pea gravel, but weight is a concern with this. Broken cement block pieces or brick pieces would work, again a weight factor. Another option along these lines is lava rock.
Typically the pros use filter media such as bioballs, plastic ribbons (biofill), plastic netting, filter foam, flexable synthetic fibers, etc. Look or search the aquaculture field for what is commonly used. Although plan on some expense if you use some purchased speciality items. You should research biofilters and biofilter sizing before going much further. Aquatic Eco-Systems catalog can get you started: aquaticeco.com
see Techtalk 95 for filter sizing info.
I would consider making the 55 g drum a two stage filter. On top of the drum or just inside, I would first have a solids filter to trap largest particulates. This would consist primarily of a removable filter foam or polyester washable 1/2"-2" thick mat. This would filter out lots of larger solids and could be removed, washed/cleaned, and replaced.
After the water is prefiltered at the top you want it to then be evenly distributed or spread out over the entire top surface area of the barrel so the water is evenly distributed to all surface parts of the biofilter to begin the downward trickling and cleaning process. Pipe the water out the bottom of the filter for a return trip back to the growing tank.
Other filter designs are also available.
Last edited by Bill Cody; 01/05/08 02:22 PM.
